Nathan has a great expert insight into these things, but the issue I've found is that his knowledge is more encrypted than laid bare in his BASIC programs, which have many score global variables with abbreviated acronym names for identifiers in block capitals.

I've twice spent over a week in failed efforts to extricate the application interface of FACEHARD from the logic that informs it, and set it aside in frustration. They are just so vexingly intermingled. I may persist at some point, but I feel it would be important to have Nathan onboard to carry his work into a more re-usable format so his know-how can see its fullest potential.

I'd be happy to hand someone the best effort I've reached at this. I was moving it to Java, as I felt it was a stone's throw from C++.

Yes, the QBasic-stuff is really nasty (as any basic - duck), I was running it through a BASICtoC converter and you may see more of the logic than of the language. (the converter is BCX-32: BASIC to C Translator by Kevin Diggins (c) 1999-2004 Ver 5.05)

Anyway I don't exactly imagined to reuse his code, but the formulas, it's not that dificult after all.

Nathan has a great expert insight into these things, but the issue I've found is that his knowledge is more encrypted than laid bare in his BASIC programs, which have many score global variables with abbreviated acronym names for identifiers in block capitals.

I have also rummaged through his code for both FACEHARD and M79 on several occasions and came away with similar feelings. It is difficult to determine how or why one arrives at the output for either program.

In comparing the output with my own data base of plate penetration testing, I find the output for M79 to be questionable. It is fine for obliquities up to and including impacts at about 30-degrees (NATO) or less. Beyond that M79 tends to diverge rather dramatically from empirical data.

What I found curious was the M79 Program does not correlate well with the actual M79 projectile at higher obliquities (or M62 for that matter). There is a huge amount of penetration testing data available from Dahlgren on both 3-inch M79 AP and 3-inch M62 APC at obliquities ranging from zero to 70-degrees. Why the M79 Program has problems with predicting capabilities for its name sake projectile at higher obliquities is therefore a bit of a mystery. But again, attempting to pin down within the M79 code what or how slope effects are being modeled or applied has proven impossible for me. It's a blackbox._________________SHUFTI CUSH

I will post my half-digested Java version of FACEHARD some time soon. I had a really nice architecture.

One thing that needs to be done is that Nathan's app just asks you to supply answers to questions that you'd wish the app to supply FOR you, e.g.: "Does the shell lose its windscreen?" I have little idea what a windscreen IS, much less the conditions under which a given one would fall off. ;)

The "windscreen" is the same thing as the "ballistic cap". The most efficient projectile nose-shape\head-shape for armor penetration is not the most efficient nose-shape\head-shape with respect to ballistic drag. As you are already aware, a thin metal windscreen\ballistic cap decreases velocity drop vs. range. See attached image.

But the windscreen has not direct effect upon perforation capability of a projectile. The indirect effect is of course maintaining lower ballistic drag such that a ballistic capped projectile (or windscreen capped projectile) will have a higher impact velocity as a F(x) of range relative to an uncapped projectile – that is if one holds all else constant aside from nose\head shape.

Why the presence – or lack thereof -- of the windscreen is included in a perforation model is therefore a bit of a mystery. I suppose it might add a tiny bit of mass. But the windscreen is so thin and so soft that its overall contribution is miniscule relative to the weight of the penetration cap and projectile body.

The only other thing I can think of that he may have been attempting to model is spin yaw effects on perforation at close range. Perforation testing is often carried out at relatively short ranges. The charge\propellant weight is adjusted to simulate impact velocity at a variety of ranges. However, sharp nosed projectiles have a large amount of spin yaw in the initial 100-200 yards of flight. After which the spin yaw tends to dampen considerably. Spin yaw (or any sort of yaw for that matter) can affect perforation capability – rather dramatically in some instances. Removal of the windscreen would often dramatically reduce spin yaw during that first 100-200yards from muzzle. But this also seems a bit of stretch considering the input parameters for the program and what the program seems to be attempting to simulate.

This is just part of some of the side by side comparisons I had run some time ago using N.Okun’s M79 program vs. both actual ballistic testing data as well as other perforation prediction algorithms.

The first figure is actual ballistic test data generated at the BRL using 2.5” Class-B armor at varying hardness levels and varying obliquity (Class-B armor being rolled homogeneous armor or RHA). e/d = 0.833. The data points are derived from actual test results for the 3” M79 AP projectile. Everything is in terms of the projectile just barely passing completely through the plate the specified impact velocity and obliquity – US Naval Ballistic Limit -- BL(N).

On the other hand, the solid lines represent predicted penetration from a post-WWII empirically based equation developed by the BRL for predicting limit velocity. This is included just for giggles. Note that the BRL equation predicted values is keyed to what was perceived at the time to be optimal plate hardness values relative to plate thickness and most probable threat. These were the hardness levels that RHA armor of the period were produced at for 2.5-inch thick plate. Moreover, US manufacturing specifications of the period were for 2.5-inch thick Class-B plates to be produced at hardness levels between about 240 to 270BHN.

The next figure represents the same BRL test data for spec armor hardness levels as well as the BRL Equation Predicted values -- again these are all stated in terms of complete through criteria BL(N). The yellow curve is the Naval Limit velocity BL(N) as predicted by N.Okun’s DOS program M79 program. As you can see there is a fair bit of contrast in the effect of obliquity on limit velocity between Okun’s predicted values, the BRL predicted values, and the actual BRL test results.